Helicobacter pylori is an important etiological pathogen of human stomach diseases, such as gastritis, peptic ulcer, and gastric carcinoma (1). In the past few years, great progress has been made in the cloning and characterization of H. pylori genes. Success of these studies stems in part from the finding that chromosomal and recombinant plasmid DNA are able to be efficiently transformed into H. pylori cells by natural competence (2-4) and electroporation (3,5). Such techniques allow the transfer of cloned H. pylori genes, manipulated in vitro, which can then shed light on the structural and functional relationships of the genes of interest. In this chapter, we describe the protocols for the isolation of H. pylori chromosomal and plasmid DNA, natural transformation, and electroporation.